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The present invention relates to a system for introducing a gaseous sample and/or calibration streams into an analytical instrument such as a mass spectrometer. More particularly, the present invention is concerned with an improved method and means for the collection and introduction of gas samples into mass spectrometers.
Mass spectrometry technique has been applied extensively to numerous applications, including in situ monitoring and effluent analysis of microelectronic manufacturing tools. In a typical mass spectrometer, a beam of electrons is used to ionize gas molecules inside the mass spectrometer vacuum chamber. These ions are directed, via electric and magnetic fields, to a detector which produces a mass spectrum. Mass spectrometers typically have a vacuum chamber or spectrometer tube into which the gas to be analyzed enters. This vacuum chamber is often operated at a pressure less than 10xe2x88x925 torr. Accordingly, in operations of a generalized mass spectrometer gas analyzer, the sample gas under test is admitted, under vacuum, into the ion source of the mass spectrometer wherein the sample gas is ionized such as by electron bombardment thereof. The resulting ion beam is then projected downstream through an analyzer which may comprise a controllable magnetic field oriented perpendicularly to the axis of the ion beam. The magnetic field thus acts to xe2x80x9cfocusxe2x80x9d or cause each specie of ion or ion fragment forming part of the ion beam to describe a definite characteristic trajectory, the radius of trajectory being dependent upon the mass-to-charge ratio of the particular ion or ion fragment specie being acted upon by the magnetic field. Stationed at the end of the ion beam path is an ion collector which, in association with appropriate electronics, is responsive to the impact of those ions and ion fragments not segregated out of the beam by the analyzer. By scanning the analyzer over a range or spectrum of magnetic field strengths, the ion collector is made to quantitatively respond to different ion mass species in the sample gas ion beam.
Variations on the above-outlined theme of mass spectrometer gas analyzer operations are also known. For instance, time of flight mass spectrometers depend upon accelerator components which accelerate the ions and ion fragments formed in the ion source. Here, the ion beam is subjected to controlled acceleration forces generated by a dynamic electric field. For a gas sample containing various charged ion species, each mass-to-charge specie of the sample has associated with it a unique flight time in the dynamic electric field, which time is measured. Accordingly, the various particles are resolved according to their mass-to-charge ratios by recording differences in their flight times in the dynamic electric field.
Other common types of mass spectrometers available include quadrupole, Fourier transform, ion-trap, and magnetic-sector. For example, in operation, the quadrupole mass spectrometer consists of an ion source, ion optics that accelerate and focus ions through an aperture in a quadrupole mass filter, an exit aperture, an ion detector and a high vacuum system. The quadrupole mass filter has four parallel metal rods. Applied voltages affect the trajectory of ions traveling down a center point between the four rods. At defined DC and AC voltages, only ions of a certain mass to charge ratio pass through the filter while all other ions are deflected out of the center. A mass spectrum is obtained by monitoring the ions as voltages are varied.
Whatever the precise mode of operations employed in mass spectrometer gas analyzers, however, all are characterized by their operations under relatively extremely low pressure, by their application of an ionizing charge to the sample gas molecules undergoing analysis and by their selective measurement of parameters which are directly relatable to mass-to-charge relationships of the ionized species of the sample gas.
An important issue relating to the use of a mass spectrometer is sample introduction. Several sample introduction techniques and sampling inlets are currently used for in situ monitoring and effluent analysis. The primary function of these inlets is to limit the amount of gas entering the mass spectrometer which, as stated above, is typically operated at a pressure below 10xe2x88x925 torr.
Commonly used inlets are made of capillary tubing, small orifices, and leak valves. Using a capillary tube is an inexpensive technique and can be adjusted to a wide range of pressures by varying the dimensions, particularly the length of the tube. The conductance of the gas sample is inversely proportional to the length of the tube. To keep the sample flow rate constant, the tube can be made longer as the pressure of the sample increases. However, the use of capillary tubing often causes a delay in response time and a mass discrimination effect, which can occur when gases of vast differences in molecular mass travel through a long tube.
Leak valves allow the user to easily adjust the flow rate of gas as the sampling pressure changes. A drawback of leak valves is the elastomer material that is often used in the construction. These elastomer materials can absorb and desorb some chemicals, which may result in a memory effect. Also, leak valves often have long sample clearing time due to their internal dead volume.
The mass spectometer with its ion source is located in the vacuum chamber. The ions from the ion source in the mass spectrometer are admitted together with large quantities of ambient gas. For this purpose, small openings with diameters of approximately 30 to 300 micrometers, or 10 to 20 centimeter long capillaries with internal diameters of approximately 500 micrometers are used. The excess gas, particularly the ambient gas, usually must be removed by means of differentially operating pump stages. In the case of commercially available mass spectrometers, there is usually one or two turbomolecular pumps and one or two mechanical pumps (sometimes called backing or roughing pumps) with a corresponding number of pump-connected chambers in front of the main chamber of the mass spectrometer to create the required vacuum.
The capillary system can cause a few problems. First, the clearing time of the capillary is not fast, particularly for a long one. For slow processes, this is not a true disadvantage. Second, the capillary is easily clogged. If a small particle gets into the capillary, it will get clogged. The particle can move along the tube, which can make the capillary impossible to unclog. It is not easy to change the capillary in most commercially available instruments.
To circumvent the problems related to the capillary tube and the leak valve, a small orifice to limit the flow of gas samples into the mass spectrometer may be used. The use of one orifice as a sample inlet works well; however, the flow rate of gas through one small orifice is relatively high due to a large pressure difference. Therefore, it is necessary for the mass spectrometer chamber to have sufficient pumping capacity for handling a high gas load. The requirement for a large pumping capacity leads to the use of an expensive, differentially pumped system.
As is already apparent from this description, the differentially operating pump stages used up to now are disadvantageous. They make it more difficult to transfer the sample gases to the mass spectrometer, make operation of the mass spectrometer complex and require the use of several costly, large high-vacuum pumps.
The cost of a differentially pumped system is considered an inhibiting factor for using the mass spectrometer as an analytical instrument. Furthermore, most differentially pumped system are custom-made and are difficult to find.
Several other patents in the prior art provide inlets for mass spectrometers. However, it is believed that none provides the unique combination of features of the present invention. U.S. Pat. No. 3,933,047 provides a mass spectrometer system with a gas sampling inlet having an inlet orifice and an outlet orifice. However, here, unlike the present invention, the port which establishes communication with the mass spectrometer is located between the two orifices. Additionally, the ""047 sampling inlet must operate under critical flow conditions which is defined as that gas flow rate through he orifices which cannot be further increased simply by further decreasing the pressure on the low pressure sides of the orifices. Finally, the orifice size and the difference in size between the orifices is not deemed critical.
PCT published application PCT/CH97/00122 discloses an arrangement for connecting a low pressure inlet of a gas analyzer. This application discloses a gas inlet that has an adjustable escape valve.
U.S. Pat. No. 5,859,433 provides an ion trap mass spectrometer that uses only a single high-vacuum pump without the need for differential pumping by means of a series of small inlet openings.
It is principally desired to provide a novel inlet for a mass spectrometer for the introduction of sample gases.
It is further desired to provide an inlet for a mass spectrometer that is capable of sampling gases at atmospheric pressure without requiring a differentially pumped system.
It is further desired to provide an inlet that is capable of sampling gases at atmospheric pressure without requiring a differentially pumped system where the sample inlet possesses the desired characteristics for effluent monitoring.
It is further desired to provide an inlet for a mass spectrometer that is capable of introducing compositionally representative samples into the mass spectrometer.
It is still further desired to provide an inlet for a mass spectrometer that provides a rapid and accurate method for continuously sampling gases at atmospheric pressures.
It is also desired to provide an inlet for a mass spectrometer that is capable of withstanding a corrosive environment.
It is still further desired to provide an inlet for a mass spectrometer that is functional over a wide range of pressures.
It is still further desired to provide an inlet for a mass spectrometer that has a short response time.
It is still further desired to provide an inlet for a mass spectrometer that does not easily clog.
It is further desired to provide an inlet for a mass spectrometer that has a minimal amount of dead volume.
Finally, it is desired to provide an inlet for a mass spectrometer that has negligible memory effects.
The present invention is a gas sampling and inlet device for a mass spectrometer. The inlet device has two orifices of different diameters used to limit the flow of gas samples into the mass spectrometer. The two orifices form a pressure reduction region regulated by a vacuum pump and needle valve. Importantly, the flow of gases through the inlet orifices is viscous flow. The sample inlet is applicable to sampling gases at various pressures, ranging from atmospheric to a few torr. The inlet allows direct sample introduction into the ion source of a mass spectrometer.
In a first embodiment, the gas sampling and inlet device for a mass spectrometer includes a hollow housing sleeve having a first end, a hollow body, and a second end, the first end has a first end cap and the second end has a second end cap. The first end cap has a small diameter orifice adapted to receive a gaseous fluid and the second end cap is sealed to a mass spectrometer. The second end cap has an orifice adapted to receive a gaseous fluid into the mass spectrometer that is substantially smaller in diameter than the orifice in the first end cap. The housing sleeve has a vacuum pump port to allow a vacuum to be created in the interior of the inlet device. The inlet device is preferably operated in the viscous flow regime. Additionally, the inlet is preferably made of all metal components. It is preferable that pressure inside the inlet is approximately 10 to 100 torr.
Preferably, the gas sampling and inlet device for a mass spectrometer includes the above, and additionally includes a hollow inner sleeve having an open inside region, coaxial to the housing sleeve. The inner sleeve has an inlet end, a hollow body, and an outlet end. The first end of the housing sleeve and the inlet end of the inner sleeve are coaxial and sealed to the first end cap. The outlet end of the inner sleeve is open such that the open inside region of the inner sleeve is open to an open inside region of the housing sleeve. Preferably, a vacuum source tube is connected to the housing sleeve at a location between the inlet end and the outlet end of the inner sleeve. Additionally, it is preferable that the inner sleeve is xe2x85x9 inch to xc2xc inch shorter than the housing sleeve.
In the above embodiments, it is preferable that the housing sleeve and inner sleeve (if applicable) are cylindrical in shape.